This invention relates to an alloy for the storage of hydrogen, and more particularly to a novel and useful Mischmetal calcium type multi-element alloy for the storage of hydrogen, which is capable of occluding a large volume of hydrogen in the form of a hydride and releasing the hydrogen readily and rapidly by application of a small amount of heat.
The term "Mischmetal" (hereinafter indicated simply as Mm) refers to an alloy which comprises 25 to 35% (by weight; hereinafter the same) of lanthanum, 40 to 50% of cerium, 4 to 15% of praseodymium, 4 to 15% of neodymium and 1 to 7% of samarium plus gadolinium and at least one element entrained by the raw material selected from the group consisting of 0.1 to 5% of iron, 0.1 to 1% of silicon, 0.1 to 2% of magnesium and 0.1 to 1% of aluminum.
As a new energy source to take the place of fossil fuels, hydrogen has recently come to attract increasing attention because of its salient advantage that it has promise of limitless supply, it is clean, it is transportable and its use as an energy source does not disturb natural cycles.
Heretofore, hydrogen has been stored in the form of gaseous hydrogen, liquefied hydrogen or metal hydride. Of the various forms of storage, the storage of hydrogen in the form of a metal hydride has recently come to attract particular attention. This is because alloys of a certain kind are capable of storing hydrogen with a density equal to or even greater than the density with which liquefied hydrogen is stored, and they are expected to enhance the efficiency of hydrogen storage. On the other hand, metal hydrides have come to be looked upon as energy conversion materials which serve to convert the chemical energy of hydrogen into thermal, mechanical or electrical energy.
The requirements which must be fulfilled by a substance to be used for storing hydrogen in the form of a metal hydride are (1) that the substance should be chemically stable and abundantly available from the standpoint of natural resources, (2) that it should be readily activated and should possess a high capacity for occlusion of hydrogen, (3) that it should possess the optimum equilibrium dissociation pressure and heat of formation suitable for the intended use, (4) that it should permit the reactions of occlusion and release of hydrogen to occur reversibly and each of these reactions to occur at a high rate of speed, (5) that it should be inexpensive and sufficiently available to promise feasibility, and so on. The transition metals such as Ti, Zr, La and Mg which have heretofore been known to produce hydrides have poor qualities for use as substances for storage of hydrogen because the hydrides of these metals are highly stable thermally and do not liberate hydrogen unless their temperatures are elevated to levels higher than 300.degree. C., for example. In recent years, alloys of Ti-Ni, Ti-Co, Ti-Fe, La-Ni, Mg-Ni, Mm-Ni and Mm-Co have been developed. However, all have defects as substances for storage of hydrogen. Of the aforementioned alloys, those of Ti, La and Mg are as thermally stable as the metals Ti, La and Mg mentioned above or require a long time for effecting occlusion and release of hydrogen. The activation effected on these alloys can hardly be called easy. In the case that one of these alloys is used for the storage of hydrogen, the metals from which the alloys are produced must be of very high purity and, in this respect, there is an economic problem. Moreover since their capacities for occlusion of hydrogen are affected by the purity of the hydrogen, the hydrogen subjected to occlusion by the alloys is required to possess a high purity.
Comparison of the Mm-Ni and Mm-Co alloys reveals that while the former alloy possesses a high equilibrium dissociation pressure despite a large capacity for hydrogen occlusion, the latter alloy suffers from a small capacity for hydrogen occlusion despite a low equilibrium dissociation pressure. The activation of the Mm-Ni type alloy requires the hydrogen pressure to be as high as 80 to 90 kg/cm.sup.2 or the treatment of activation to be performed for an excessively long period or to be repeated a number of times. This alloy consumes much time in occluding or releasing hydrogen.
As improvements over these alloys, there have been invented an alloy of the composition MmNi.sub.5-x Co.sub.x (U.S. Pat. No. 4,147,536), an alloy of the composition MmNi.sub.5-x A.sub.x (U.S. Pat. No. 4,222,770), and an alloy of the composition MmNi.sub.5-x Cr.sub.x-y A.sub.y (U.S. application Ser. No. 192,809, dated Oct. 1, 1980). The alloy of the general formula MmNi.sub.5-x Co.sub.x approximate the Mm-Co alloy in equilibrium dissociation pressure and the Mm-Ni alloy in capacity for occlusion of hydrogen. The alloy of the general formula MmNi.sub.5-x A.sub.x, though superior to known alloys in properties such as speed of hydrogen occlusion, activation and equilibrium dissociation pressure which are essential for the occlusion of hydrogen, falls short of satisfying requirements for practical application. The alloy of the general formula MmNi.sub.5-x Cr.sub.x-y A.sub.y enjoys a peculiar feature never attained by the conventional alloys, i.e. the fact that it exhibits a constant dissociation pressure over a wide range of hydrogen/metal atom ratios, in other words, it possesses a small flatness factor.
Another alloy of the general formula Mm.sub.1-x Ca.sub.x Ni.sub.5 (U.S. Pat. No. 4,096,639) enjoys a low cost of production but exhibits a high equilibrium dissociation pressure. It is thus difficult to use as a material for the occlusion of hydrogen.
As described above, alloys possessing properties suitable for the purpose of hydrogen occlusion have been developed in rapid succession. They have much room for improvement and have a common drawback that the cost of production is high.
In the circumstance, an alloy which is excellent in properties for the purpose of hydrogen occlusion, inexpensive to produce and fully feasible is in demand.